Generally, to effect a flow of air through the radiator core or heat exchanger sufficient to effect heat transfer of the coolant, a fan assembly attached to the engine is provided on the motor side of the radiator to draw air therethrough. The rotation of the fan blades is directly related to the RPMs of the engine. Thus, the slower the rotation of the fan, the smaller the quantity of forced air flow.
Most cooling requirements of an internal combustion engine vary with the speed of the vehicle. Generally, cooling is most important 5% to 10% of the time during idling, slow speeds, or climbing steep grades, since natural convection at vehicle speeds in excess of 20 to 30 mph is sufficient to effect air flow through the radiator to maintain proper operating temperatures. During normal operation, the air passing through the radiator by the vehicle's forward motion is sufficient to keep the engine cool; no fan is necessary.
Currently, to help increase fuel economy on trucks, Original Equipment Manufacturers (OEMs) have used both engine-driven on-off or modulating fan clutches. Obviously, these types of fan drives require up to 30 hp to rotate when they are engaged, reducing overall fuel economy and engine efficiency. Furthermore, the fan drives: (1) must be turned directly by a belt from the engine; (2) restrict the modifications that can be done to the engine and chassis because the engine, radiator and fan must be adjacent and on the same plane; (3) are prone to maintenance problems because the fan or fan hubs are in constant rotation; (4) reduce life expectancy of belts and water pumps because of the strain the fan places on them when it is engaged; (5) cause vibration because they are attached to the engine; (6) are expensive to install; and (7) tend to overcool the engine, creating sludge deposits that cause premature engine wear.
It has been the practice of Original Equipment Truck Manufacturers to overcool their engines because:
(1) They offer prolonged warranties. During this coverage they want to make certain that if the vehicle comes into contact with a harsh environment that contains contaminants (dust or dirt), partly clogging the radiator, there is enough air flow to maintain proper engine temperatures. The OEMs over-design the CFM requirements to compensate for improper maintenance. PA1 (2) Economically, it is less expensive to manufacture vehicles having the same components, thereby reducing the number of different parts, certifications, and designs. OEMs build trucks with the same components, even though they may be used for different purposes, and therefore require different air flows. For example, a stationary application such as a sanitation truck which uses its engine at high idle to load and unload has a greater air flow demand than an over-the-road truck that benefits from ram air cooling.
Electric fans for engine cooling have been successfully used on automobile engines for years; however, on truck applications that benefit from ram air cooling (air forced through the radiator by the vehicle's forward motion), both the Society of Automotive Engineers and the Original Equipment Manufacturers have rejected their use, since it is their position that a multiple of electric fans are inadequate and cannot provide the same cooling capabilities of belt-driven on-off, viscous, or direct-drive fans that are currently used.